Fuel cell power plants are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is known that thermal and water management of fuel cells typically includes circulating a coolant through the fuel cells to remove heat during operation of the cells, and to add heat during a start-up procedure. It is also known to utilize porous water transport plates adjacent porous catalyst support layers in fuel cells utilizing a proton exchange membrane (“PEM”) electrolyte.
The porous water transport plates typically define coolant and reactant stream channels on opposed sides of the plates for transporting a coolant through the cell, and for providing the reducing fluid fuel and oxidant reactant streams to the cell anode and cathode catalysts. Product water generated during operation of the fuel cells and any condensed water from saturated reactant streams typically moves through such porous water transport plates and into coolant channels of the plates to be transported out of the fuel cell with the circulating coolant.
Operation of such fuel cell power plants in conditions where ambient temperatures decline to below the freezing temperature of water gives rise to significant thermal and water management issues. For example, coolants including traditional antifreeze solutions of ethylene glycol and water or propylene glycol and water will diffuse through the porous water transport plates and catalyst support layers to be adsorbed by and poison the fuel cell catalysts. Additionally, such antifreeze solutions have low surface tensions resulting in wetting of any wetproofed catalyst support layers, thereby impeding diffusion of the reactant streams through the porous support layers to the catalysts. Those antifreezes also have high vapor pressures resulting in evaporation and loss of traditional antifreeze solutions in fuel cell exhaust streams or fuel cell power plant support systems.
It is known to utilize alternative antifreeze solutions to minimize the described limitations of those traditional antifreeze solutions. In particular, a variety of “direct antifreeze solutions” are disclosed for use in fuel cell power plants in U.S. Pat. Nos. 6,316,135, 6,361,891, 6,365,291, 6,416,891 6,432,566, and 6,461,753, and in U.S. patent application Ser. No. 10/194,122, which Patents and Application are owned by the owner of all rights in the present invention. The direct antifreeze solutions are in essence organic antifreeze solutions that are non-volatile at fuel cell operating temperatures. Because such direct antifreeze solutions are non-volatile, they are circulated through porous water transport plates in “direct fluid communication” with the fuel cell catalysts without volatilizing to contact and poison the fuel cell catalysts, or to leave the plant in exhaust streams.
It has been determined however, that while the direct antifreeze solutions enhance performance of such plants, the performance of the plants is still not the same as performance with pure water as the coolant. Additionally, operation of fuel cell power plants with such direct antifreeze solutions still requires active water management to humidify reactant streams and to remove fuel cell product water. That is typically achieved through a coolant loop wherein the fuel cell product water is mixed with the direct antifreeze solution and water in the coolant solution humidifies the reactant streams. As disclosed in the aforesaid U.S. Pat. No. 6,365,291, controlling concentrations of the direct antifreeze solution within a complex coolant stream requires a concentration control system involving power from the plant for active pumping of fuel cell product water, and increased volume, weight and cost of the system components.
Circulating fuel cell product water as part of a thermal management system also gives rise to substantial difficulties in storing significant volumes of water when a fuel cell power plant is shut down in ambient temperatures below the freezing temperature of water. For example and as disclosed in U.S. Pat. No. 6,562,503, which Patent is owned by the owner of all rights in the present invention, a large volume of fuel cell product water utilized as a fuel cell coolant may be stored in a freeze tolerant accumulator along with a water immiscible fluid antifreeze solution that is used to thaw frozen water within the accumulator during a start-up procedure. Such a freeze tolerant fuel cell power plant also gives rise to increased parasitic power from the plant, and increased volume, weight and cost.
Consequently, there is a need for a fuel cell power plant that minimizes components required for circulating a coolant, and that minimizes a total volume of water within the plant.